(light music) - [Bret] One of the best ways to enjoy the Land of 10,000 Lakes is by boat.

But what impact does our use of powerboats have on the lakes?

Researchers at the St.

Anthony Falls Laboratory, an institution specializing in the study of fluid dynamics, decided to investigate.

(gentle music) In recent years, the sport of wake surfing has been gaining in popularity, and with it, the watercraft that makes the sport possible: wake boats.

- So these boats are designed for surfing, and by that I mean somebody will get on a small surfboard, and right off the back of the boat, they surf on the big wave that's produced.

I have never tried surfing myself.

I would love to try it, actually.

As an older person, that one of the advantages of the sport is that it's done at a slower speed.

- [Bret] At first glance, the crafts used for surfing may look rather ordinary.

- Wake boats are really similar to typical recreational boats, but there are some very important distinguishing features.

The first is that they can take water on board.

They have ballast tanks that the operator can throw a switch, and these tanks fill up with water, and they actually kind of double the weight of the boat.

So a boat that might weigh 3 or 4,000 pounds all of a sudden weighs, 8, 9, or 10,000 pounds.

And that makes a big difference in how much water it displaces.

And because it is so heavy, it needs a big engine.

So the motor on board, these can range from 300 to 600 horsepower, which is far greater than boats.

At least like when I grew up, we were happy to have 85 horsepower in a boat to ski behind.

These boats are big and powerful.

- There's also things called surf tabs, which are little tabs on the back of the boat that can be deployed that help shape the wave and make it so it's a little smoother so that you can ride on it on the wave.

And then there's also things called wedges or hydrofoils, and those are on the back of the boat and can also be deployed to help pull that stern into the water more to create that surfable wave.

And the key factor about these boats is the speed at which they're moving.

So we call it the mode of operation, and those boats are going generally at two different modes of operation.

- If you've been on a boat, you've water-skied or you've tubed, the boat is hydroplaning.

It's going fast enough where lift forces are generated, much like an airplane wing, and the boat rides up on top of the water and sits up there, with very little contact between the water and the boat.

Another condition is if you're just going slowly, you're trolling or you're pulling away from your dock, we call that displacement.

The hull is actually supported by buoyancy forces holding the boat up.

- But then you have this mode of operation that wake boats are in during surfing, which is called semi-displacement mode.

And that's that transition period between slow displacement mode to fast planing mode, and it's this plowing mode and that speed is what's also, in addition to the hull and the weight and technology, it's that speed that is really helping to create these big waves.

- [Bret] As surf waves become more common on the lakes of the Upper Midwest, interest in the boats producing them has grown.

- We actually started getting a lot of phone calls and emails kind of around the 2019 timeframe, asking if we're studying or know anybody that's studying the issue of wake boats.

- And a lot of the questions we were getting asked is, is there any data out there about these boats, the size of waves that they're creating?

There was not much data out there in the scientific literature.

- So there was a real gap there.

And as University of Minnesota, that's our role.

We're a land-grant research institution.

We see our role as doing research that impacts Minnesota or the Upper Midwest.

And so we kind of threw our hat in the ring and got to work.

- [Bret] Phase one of the study examined a noticeable boat-created wave.

- I'm sure we're all familiar with that V-shaped wave that comes off the back of a boat.

Those are the waves that you see when you're standing on shore and waves kind of come crashing into the shoreline.

That's called a divergent wave, or the divergent waves.

How we tested that in the field was we had sensors at various depths and distances from shore, and then we would run boats at also certain distances, and we would measure those waves coming into shore past the sensors.

And the sensors were basically measuring wave height.

From wave height, you can do some calculations and also get wave energy and wave power for the entire wave train.

So we tested four different boats.

Two of them were wake boats, and then we tested two additional boats that would be non-wake-surf boats.

Again, typical boats that have been around for decades.

- [Bret] Phase one compared boats under typical operating conditions at a distance of 100 feet from the boat and found that the wake-surf boats produced maximum wave heights that were two to three times larger, total wave energies that were three to nine times larger, and maximum wave powers that were 6 to 12 times larger than the non-wake-surf boats.

- So basically the same time we published phase one, we were already starting phase two.

- [Bret] The second phase of the study took a more in-depth look at how boats were interacting with lakes.

- We're now looking at what's going on in the water column, so the water column being from the lake bed up to the water surface, so that entire column of water.

And what we were looking at was three additional hydrodynamic phenomena, and that being what's called the propeller wash.

Most boats in operation, they're being pushed by a propeller that's rotating, and off that propeller, you have all this turbulent water and exhaust bubbles, and that's getting pumped down and pushed down into the water column.

Then there is what's called bow and stern waves on a boat that's moving.

As the boat is moving, the bow is pushing water down and forward, and at the same time, the stern is creating what we call a stern wave, where the water is rebounding and moving backwards.

Then you have the third hydrodynamic phenomenon that we looked at, which is called the transverse waves.

And these, if you are sitting in the boat, you're a passenger, and let's say, you're looking out behind the boat, you're gonna see these big rolling waves that almost look like they're moving with the boat.

Those are transverse waves, and these are actually really large circular waves in the water that are following the boat.

(light music) - The findings of this study are really interesting, both for the wake-surf boats and for non-wake-surf boats.

For non-wake-surf boats, we saw that even boats that aren't surfing can be felt by the bottom of the lake, depending on how deep you are.

So even if you're hydroplaning across the lake in your ski boat or your fishing boat, the lake knows you're there.

You're creating velocities, and if you're in too shallow of water, the lake can actually be disrupted.

- [Bret] As part of their phase two study, researchers made operational depth recommendations for both traditional powerboats and wake boats.

- Boats like the ones we tested, when you're on plane, we recommend maintaining 10 feet of water or greater to minimize impacts to the lake bottom.

When you're in that slow displacement mode, so just slow, leisurely cruising around the lake, again, 10 feet of water or greater is what our recommendation is.

For wake boats, when you're in surfing mode, so again, that's slow kind of plowing mode, we recommend 20 feet of water or greater when surfing.

- So some of our data that is most compelling is video data.

We put GoPros down on the bottom of the lake.

And what you see under surfing conditions in too shallow of water, 15 feet, 10 feet, is the lake bottom absolutely erupting, getting blown apart by the velocities that are produced.

(gentle music) And we're actually quite concerned about this in our research group.

We don't exactly know all the impacts of that, but we know that the bottom is completely eroded, suspended in the water column.

We see vegetation flying by the frame of the video, impacts that are very severe.

So you see vegetation pass by that probably took one or two or three years to establish itself, and in one boat pass, it's gone.

What does that mean to our lakes?

It's something that we need to ask ourselves, both the researchers but also the citizens that are operating these boats need to think about what's happening underwater when I use my watercraft.

I'm Minnesotan, so I'm really proud of our lakes.

I think it's one of our big legacies.

Nowhere else in the lower 48 do we have lakes that are so clear, so natural.

And so if we lose that, that's a real problem.

- So for phase one, we did not look at any environmental impacts.

It was simply looking at the wave characteristics.

For phase two, we did collect and start to look at potential environmental impacts, which is that disruption to the lake bottom.

And again, sediment resuspension.

And why that's a concern is because if you have sediment resuspending into the water column, specifically really fine sediments, they can stay in the water column suspended for long periods of time.

- We call that turbidity.

It's how much light can penetrate the water column.

We think about how deep can I see down.

When I look on my dock, can I see the bottom or not?

Or, no, I can only see a couple feet?

- So turbidity can disrupt vegetation growth.

It can make the water warmer because there are particles in that water that can absorb the heat.

And if you have warmer water, you could potentially have decreased dissolved oxygen, which can affect fish, plants, and such.

Another thing is nutrients.

There's a lot of nutrients in our lake bed.

Disturbing that lake bed and potentially releasing those nutrients, you could end up having excessive algal growth or plant growth.

If we think about sight-feeding fish and now we have increased turbidity, these fish might not be able to see as well and feed as well.

There's just a lot of things that are kind of like what-ifs, and we know from the literature that it's a possibility.

- [Bret] The studies become a hot topic to lake users since their release.

As they continue to gather data, researchers hope the study can help Minnesotans determine a path forward regarding their lakes and their boats.

- I hope that our study and our results get taken seriously and can help inform science-based guidance, because the ultimate goal here, I think, from most people that either live on lakes or recreate on lakes or just enjoy Minnesota resources, is let's preserve what we have.

They're a gem.

We have amazing lakes here in Minnesota.

Wake boats and the sport of wake surfing are likely not going anywhere.

They're here to stay, and so we just have to come up with a way to use the resource so that everybody can use it.

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